Electrophotographic methods for creating copies of documents have been incorporated into copier and printer devices for use in a wide range of applications. Machines incorporating such electrophotographic techniques have found use in offices as optical copiers of documents or document printers which receive the document image electronically.
The basic aspects of the copying process known as xerography are well known to those of skill in the art. Known electrophotographic machines use an electrostatically charged photosensitive medium or photoreceptor to receive the document image. The photoreceptor has a uniformly distributed electrostatic charge placed thereon by a conventional charging corona. A latent electrostatic image is developed on the photoreceptor by selective exposure of charged areas the photoreceptor medium to light from an image source. A variety of techniques may be used to create this latent image, including use of a stroboscopic flash and appropriate optics, a raster scanned laser, or a linear scan mechanism which moves a light source, optical elements, or both in synchronization to scan the photosensitive medium with the image of the original.
The latent image on the photoreceptor passes adjacent a source of toner materials which are pulled by an electrostatic force onto the latent image. This creates a developed image on the photoreceptor. As is knwon to those skilled in the art, these toner materials are preferably plastics which melt at a predetermined temperature and have appropriate color characteristics once they are melted.
The photoreceptor carrying the electrostatic developed image then contacts an image receptor which, in most common applications of xerography, is a sheet of paper. The toner transfers from the photoreceptor to the image receptor by electrostatic charging technique.
The image receptor now carries the toner particles and thus images the document to be copied. The image receptor passes through a heat station or fuser where the transferred toner is heated to melt the toner particles. Duration and temperature ranges to fix the image in the paper are well known by those of skill in the art.
As may be appreciated, residual toner remains on the photosensitive element after the developed image transfers to the print receptor. Such residual toner must be removed from the developing position of the photosensitive element prior to re-exposing the element to a subsequent image. Known cleaning stations include a cleaning blade which wipes residual toner from the photosensitive element into a trough. Many of these known printing machines employ an auger apparatus to convey the toner from the trough to a remote storage receptical. Such mechanisms for handling waste toner increase the complexity of the machine and require laterally remote space for storage of the waste toner. Increased handling of toner increases opportunities for toner dust contamination of other copier components.
The technique briefly described above for producing monochromatic copies may also be followed for producing color copies using a process similar to conventional three color printing. The color copier creates separate latent color component images, or color separations, on the photoreceptor by exposing the photoreceptor through appropriate color filters. Each separate color component image is developed by a toner having the appropriate color characteristics. The developed color component image is transferred to the image receptor or paper in sequence to create a composite color image. The paper carrying the composite image of three toners is passed to a fuser in a conventional manner to fix the image onto the image receptor.
As may be appreciated, the electrophotographic technique for color copies requires three separate exposures of the image on the photoreceptor to develop the latent color separated images. In color electrophotographic machines three distinct color separated images are sequentially exposed on the photoreceptor medium and developed with an appropriately colored toner material. These developed color separated images are subsequently superimposed on each other and transferred to the final print receptor, such as a sheet of paper. In the art of color electrophotography it is known to directly construct the composite color image on the final print receptor medium or, alternately, to construct the composite image on an intermediate transfer medium from which the composite color image is transferred at one time to the final print receptor. The preferred embodiment of the present invention uses an intermediate transfer medium although same is not required to practice the invention described herein.
Various improvements in the art of electrophotography permit the copier and printer machines to become smaller and more compact. One feature permitting a more compact size for such devices was replacing the cylindrical drum with a horizontally disposed flexible belt driven by longitudinally spaced rollers. Toner modules horizontally spaced adjacent the photoreceptor include hoppers for holding a supply of toner material and decorator rollers for placing the toner adjacent the charged photoreceptor. In a monochromatic copier, only one such toner module was necessary. Color copiers require at least three such toner modules, and some known copiers include a fourth module for black toner rather than using a "process black" made by combining the three color toners.
Use of a photoreceptor belt in a copier permits the copier or printer to have a compact size. The step from a rigid cylindrical drum to a flexible belt was one of the major developments in electrophotography which allowed a reduction of volume occupied by machine. Use of flexible photoreceptor belts in monochromatic electrophotographic print engines has been common for a number of years. Such compact copiers are conveniently used in offices having limited space for locating a copier. Also, such compact laser printers have a lower price and have gained acceptance as printer devices for use with microcomputers and word processing systems. However, the move toward size reduction has, heretofor, led to several drawbacks.
Until recently, practical full color electrophotographic print engines required the use of rigid cylindrical drums carrying the photoreceptor and critically machined mechanical parts in order to maintain registration in the composite developed color images as the individual separated images are placed over each other. There are relatively early patent disclosures of full color electrophotographic print engines employing photoreceptor belts, for example U.S. Pat. No. 3,999,987. However such machines did not indicate practical schemes for synchronizing movement of the photoreceptor belt and the final print receptor in a manner which assures adequate registration of the separated color component images.
Even in the early disclosures of full color electrophotographic print engines employing belts, the toner modules used for developing the latent electrostatic images were disposed beneath the photoreceptor belt. The decorator brushes contacting the photoreceptor belt were pointed in a generally upward direction. This is because there was no known practical downwardly pointing toner development module available.
A substantial step forward in reducing the size of full color electrophotographic print engines was made in the machine disclosed in U.S. Pat. No. 4,652,115 to Palm et. al. This machine architecture discloses a practical downwardly pointing toner development module in a full color electrophotographic print engine employing a belt mounted photoreceptor surface and an intermediate transfer medium, also mounted on a flexible belt. A practical control scheme for achieving a registration of sufficient accuracy to provide non-smeared full color copies using process colors is shown in U.S. Pat. No. 4,652,115.
It will therefore be appreciated that earlier machine architectures employing cylindrical drums took up a significant amount of space due to the large size of the drum. In such machines the toner development modules were generally disposed around the lower portion of the machine in order to have a significant upward component to the direction in which the decorator brushes pointed, for the reasons noted here and above. Transfer of the developed image from the photosensitive element to the ultimate print receptor was normally laterally disposed in such machines thus increasing the width of the print engine.
The use of photoreceptors mounted on flexible belts was a major step in size reduction of such machines. The conventional architecture for these machines located the expose station above the belt but the toner modules were disposed below the belt for the reasons noted above. In such machines, the paper path was located below the volume occupied by the toner development modules thus requiring descending vertical layers of expose station, photoreceptor belt, development modules, and paper path. However, clearing paper jams from the machine generally requires the user move and then replace such components adjacent the paper path. This exposes the user to heated elements and toner in the copier. Improper repositioning of the components prevents the machine from operating and leads to user frustration.
In the full color architecture disclosed in U.S. Pat. No. 4,652,115, the toner development modules are located above the upper surface of the photoreceptor belt thus reducing the height of the overall architecture by allowing both the exposure station and the development modules to be positioned above the photoreceptor. This allows the paper path to be located in close proximity to the underside of the photoreceptor belt.
However, in order to maintain the small volume of such machines, it is necessary that the printed output from same (whether copying machines or laser printers) be deposited face up in the output tray. If it is desired to modify the conventional construction of a compact office copier to produce face down copies, additional height must be added to the machine for a wraparound paper path to invert the copies provided to the machine's output tray. This has been adopted in the past, but requires the space for a complete loop around paper path to deposit the copies face down, and such a longer paper path increases the opportunity for paper jams. Also, machines printing face up with a downward directed oil fuser require complicated components to provide uniform application of oil without dribbles or spots of oil on the copy paper.
It is known to those skilled in the art that it is highly desirable to provide a compact electrophotographic printing machine which will produce face down copies so that copies being printed from a conventional computer file or copies in their natural order from an original document will be in their proper collated sequence when the printing job is finished.
Therefore there is a need in the art to provide an architecture for an electrophotographic print engine which, while handling conventional office standard types and sizes of paper, will address the deficiences of compact and full size copiers or printers. Such machines will occupy a smaller volume than heretofor possible in electrophotographic print engines, provide more direct and convenient user access to the paper path and collect waste toner in gravity fed containers. Horizontal application of toner to the vertically disposed developing position permits simpler toner modules, and the oil fuser for face down printing is less complicated.